1. Field of the Invention
The present invention relates to a viewfinder device for a camera. More particularly, the present invention relates to a viewfinder device in which a target indicia or frame indicia for a photographic field to be framed can be observed clearly with an object image.
2. Description Related to the Prior Art
A viewfinder device is incorporated in a compact camera or a lens-fitted photo film unit in which photo film is pre-contained in a state as goods for sale. There is a widely used type called a reversed Galilean viewfinder device. Examples of improved versions of reversed Galilean viewfinder device are an illuminating window type, an Albada type and the like, which are characterized in viewfield indicia rendered observable for indication of a photographable range in the field of view. However the illuminating window type and the Albada type have a shortcoming of high cost with their complicated structure.
There is a prior document JP-A (Japanese Patent Laid-open Publication No.) 52-141637, in which an objective lens element or a transparent plate behind the objective lens element is provided with frame indicia for indicating a viewfield. Micro lenses ("convex lens optical elements") for adjustment of diopter are formed integrally with either an eyepiece lens element or the transparent plate, and renders the frame indicia observable as viewfield frame image. There is an advantage in the prior document in low cost, because the simplest one of its examples has the indicia on the objective lens element of plastics, and the micro lenses formed with the eyepiece lens element of plastics.
In the viewfinder device disclosed in JP-A 52-141637, the micro lenses form erect virtual images of the indicia in a position near to the position of a virtual image of an object. There is no problem when only the object image is observed through the objective lens element and the eyepiece lens element, or when only the indicia image is observed through the micro lenses. However, while the indicia is observed, part of the object image is also seen through the micro lenses as well as through the objective and eyepiece lens elements. Part of light flux from the object is passed through the micro lenses and refracted thereby the more to the extent of the power (refracting power) of the micro lenses. There occurs a difference in diopter between the object image of light flux passed through the objective and eyepiece lens elements, and the object image of light flux passed through micro lenses.
In general, the photographers aiming through a viewfinder device make their eye target an object to be photographed. It follows that each photographer observes the object image with advertence, but observes images of the frame indicia only incidentally, as his eye is focussed sharply on the object images, but less sharply on the frame indicia images. If he focuses his eye sharply on the frame indicia images through the micro lenses, the object image comes out-of-focus to his eye. It is difficult to check whether the object image is acceptably framed with reference to the frame indicia.
Such a problem is more serious with a target indicia than that of the frame indicia: The target indicia is formed in the center of the objective lens element, while a micro lens is formed in the center of the eyepiece lens element for rendering the target indicia observable. The micro lens refracts the light flux from the center of the object to the excessive extent, and changes the position of the object image in unwanted fashion from a desired position. There occurs a difference in position between the target indicia image and the object image. It is impossible in JP-A 52-141637 to position any micro lens on the optical axis. No center can be indicated in the viewfinder device with such a micro lens.